NETT-TO-GROSS: another pointless discussion.

NETT-TO-GROSS: another pointless discussion.

Juan Cottier Juan Cottier
Director & Lead Geologist at MMbbls Limited

"Il semble que la perfection soit atteinte non quand il n'y a plus rien à ajouter, mais quand il n'y a plus rien à retrancher."

(It seems that perfection is attained not when there is nothing more to add, but when there is nothing more to remove). Antoine de Saint-Exupery - French writer.

And the very first thing that should be removed is NTG.

A Quick Note on Spelling: I have always used, and will continue to use, nett with 2 "t"s because (1) I’m British, (2) I’m of a certain generation and (3) nett is an adjective. I also know a Dutch geologist of a different generation who refers to “nett over gross” which to my ear has a very refined elegance to it. I also acronymise to NTG because I'm a sucker for key-words.

“Cathago delenda est” … Following the 2nd Punic War where Hannibal scared the “excrementum” out of the Romans for 15 years the Roman Senator, Cato the Elder would end each and every one of his speeches, regardless of relevance, with:

“Ceterum censeo Cathaginem esse delendam”

(Furthermore, I consider that Carthage must be destroyed). And it was every speech; public and private, military and domestic, to the point where Cato was considered as a bit of a weirdo, obsessed, deranged, a lunatic shouting into the wind. NTG is my Carthage. I do at times feel like that lunatic shouting into the wind, but I will not be silenced until the edifices have been destroyed, the nett-to-grossers have been sold into slavery and the petrophysical fields have been sowed with salt to ensure nothing grows again.

I’ve bored a lot of people over the last 15 years regarding the non-existence of NTG as a real property and the damage it does to understanding a reservoir. Some folk disagree, some violently, most have never considered removing NTG as an option, and some are in complete agreement. It is easy to find forum discussions on NTG, here on LinkedIn for example, and it’s very common to find a bold statement such as “NTG is a very simple property used in fundamental reservoir description that represents the effective rock and is defined as” … followed by 3 rambling paragraphs.

In 2011 I emailed 25 people; graduate geologists to VPs and asked them, in their own words, to define NTG. The replies were varied and rambling. So I developed my own varied and rambling conversation:

  • Cato - What is NTG?
  • Carthage - Well, that’s very simple, it’s a ratio to determine effective rock.
  • Cato - Effective for what?
  • Carthage - Well for hydrocarbons to flow, of course.
  • Cato - Gas?
  • Carthage - Well I was thinking of oil, but yes, gas also.
  • Cato - It’s the same thing is it?
  • Carthage - Well no, but viscosity is easy to correct for.
  • Cato - Your NTG is corrected for fluid viscosity?
  • Carthage - ……
  • Cato - And effective to flow? What does that mean?
  • Carthage - I don't mean to patronise, but ...
  • Cato - That's OK, go ahead.
  • Carthage - But effective means the ability for fluids to flow through a reservoir.
  • Cato - Like permeability? So NTG is a function of permeability?
  • Carthage - Yes … well … sometimes … it can be. Yes. Or Vshale.
  • Cato - Vshale is a measure of the ability for fluids to flow through a reservoir?
  • Carthage - Absolutely! Well, let’s say it’s a proxy. Vshale is pretty easy to calculate.
  • Cato - So NTG is defined as a ratio to determine effective rock using a pretty easy to calculate proxy for permeability.
  • Carthage - Well no … but …
  • Cato - Permeability determines our effective flow?
  • Carthage - Yes.
  • Cato - Not relative permeability?
  • Carthage - No! …. Errr … Yes, actually yes, good point.
  • Cato - So like a mobility? A sort of “effective mobility”? Relperm and viscosity?
  • Carthage - Yes ... sort of ... that might work … except that …
  • Cato - Except that relperm is relative to Sw and Sw changes through time?
  • Carthage - Errrr
  • Cato - So if NTG represents effective reservoir then it can change with time?
  • Carthage - Well it's more initial Sw, before we start producing.
  • Cato - So NTG is defined as a ratio to determine a sort of effective rock using a pretty easy to calculate proxy for relative permeability, such as Vshale, before we start producing?
  • Carthage - No! No, that’s just ridiculous.
  • Cato - I agree ...... I don't suppose you know where I can buy salt by the tonne.

We forget where NTG came from a method developed 100 years ago in an attempt to understand 1D well data. A method we no longer need. We also forget that the prime directive of a Petroleum Geologist is to understand the distribution of pore-space, and NTG prevents us from doing that. It prevents us from including potentially contributing facies. I appreciate that by adding in poorer and poorer rock we have the concept of “more-and-more of less-and-less” but, without wishing to state the obvious, we are using computers, it’s no more effort.

In fact it’s less effort … I have lost count of the tech discussions I have been involved in where for the first hour the petrophysicist explains how they have made-up a definition of nett rock (I kid not, I have seen ppms of Thorium used) and then the geologist (usually a geomodeller) takes the next hour to explain how they have made-up a method to distribute the made-up nett rock. That has given us "made-up squared". In the words of Dostoyevsky:

“Lying to ourselves is more deeply ingrained than lying to others.”

Or for those of you in Scotland who might prefer some Irvine Welsh:

“There is that horrible, beautiful moment, that bitter-sweet impasse where you know that somebody is bullshitting you but they're doing it with such panache and conviction ... and they are saying exactly what you want to hear, at that point in time.”

NTG cut-offs are arbitrary (an early, poorly-informed, heartless selection process where some children most definitely get left behind) and the 2D and 3D distribution of NTG can be borderline absurd particularly in the hands of the overly-enthusiastic. We also have the ludicrous world of "petrofacies" ... the attempt to right a wrong by repeating the process in more detail.

Let's finish with a discussion between two simulation grid-cells:

Nett Cell - Hey! You!

Non-Nett Cell - Me? What?

Nett Cell - Can you take some of this off me, I’ve got 8000m3 of reservoir fluids.

Non-Nett Cell - Sorry mate, can’t help you there.

Nett Cell - What? Why not?

Non-Nett Cell - I’m non-nett mate. Always have been, always will be.

Nett Cell - What? But you're 1.99mD perm.

Non-Nett Cell - Sure am mate, and with a cut-off of 2mD that makes me quite clearly non-nett.

Nett Cell - But I’m 2.01mD … were less than 1% different … and I’m doing all the heavy lifting here … and you’re not even going to help? It’s alright for those 400mD lads up there in the delta-plain fluvial-channel or the upper-shoreface facies but us lower-middle-shoreface guys … it’s tough. And then you’re doing nothing. Come on. Please.

Non-Nett Cell - Sorry mate, would love to help ………… NOT.

Nett Cell - What about shifting some pressure then? No fluids. Just transmit some pressure?

Non-Nett Cell - Are you f’in’ deaf mate!? I - AM - NON - NETT! I’d get into all sorts of bother. The SPWLA guys would be down on me like a tonne of rocks. We can’t have cells just moving fluids and transmitting pressure just because the poroperm characteristics, the relperms and the differential pressures are favourable. Sweet Jupiter! Think about it, mate!

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